1. Field of the Invention
This invention relates to a silicon nitride circuit substrate and a semiconductor device using the same, particularly to a high thermal conductivity silicon nitride circuit substrate having improved mechanical strength and thermal cycle resistance and excellent radiation characteristic, and a semiconductor device using the same.
2. Prior Art
In recent years, as a ceramic plate which is one constitutional element of a circuit substrate, there has been reported a ceramic plate mainly comprising silicon nitride having not only high mechanical strength and heat resistance, but also high thermal conductivity in Japanese Provisional Patent Publication No. 135771/1994.
In the prior art, a direct joining method for joining a ceramic plate and a metal circuit plate without using a binding material such as a wax material. This method is a method in which an eutectic compound of components contained in a metal circuit plate or components of a metal circuit plate and a ceramic plate is generated by heating and the eutectic compound is used as an adhesive to effect joining.
This joining method is effective for oxide ceramics such as alumina, but it is not effective for nitride ceramics because the wettability of an eutectic liquid phase is extremely low, whereby the joining strength to a metal circuit plate is insufficient. Further, the present inventors have studied to clarify that among nitride ceramics, with respect to aluminum nitride (AlN), when a ceramic plate is subjected to heat treatment in oxygen-containing atmosphere such as air, an oxide layer (Al.sub.2 O.sub.3) having good wettability to an eutectic liquid phase is formed on a surface thereof, but this method is not effective for a silicon nitride ceramic plate.
Therefore, when a silicon nitride ceramic plate and a metal circuit plate are joined by a conventional direct joining method, joining strength is lowered, and when a semiconductor element is actuated to impose repeated thermal cycles on a joined portion, cracks are generated on a ceramic plate in the vicinity of the joined portion to give a circuit substrate having low thermal cycle resistance.